Ease of SN1 reactions among these compounds upon treatment with aq. NaOH will be in the order as: 
a) Iodoethane > Chloroethane > Bromoethane 
b) Bromoethane > Chloroethane > Iodoethane 
c) Chloroethane > Iodoethane > Bromoethane 
d) Bromoethane > Iodoethane > Chloroethane